1. Field of the Invention
The present invention relates to a process for the removal of carbon monoxide from a gas comprising a minor portion of carbon monoxide and a major portion of an inert gas, such as nitrogen.
2. Description of Related Art
Conventional membrane systems employed to produce gaseous nitrogen from air typically produce high purity nitrogen gas. The nitrogen purity level can be greater than 99 volume percent. The flow rate at high purity levels, however, tends to be too low to be useful because energy requirements escalate significantly with increasing purity levels of nitrogen. Therefore, this method of producing high purity nitrogen gas is not very efficient.
Alternatively, the membrane system can be operated more economically by increasing the outlet flow rate of the purified nitrogen. The effect of a higher nitrogen outlet flow rate is that higher concentrations of oxygen, as well as other impurities, are entrained in the xe2x80x9cpurifiedxe2x80x9d nitrogen gas stream, thus producing a nitrogen gas stream of lower purity. If the membrane system is operated at a high outlet nitrogen flow rate, the most objectionable impurity in the nitrogen gas stream is oxygen. Oxygen is harmful in most uses of the nitrogen as an inert gas because oxygen is an oxidizer. Therefore, the presence of oxygen is not desirable in an inert gas environment or in a gas to be used to produce a reducing atmosphere, which is required in many applications such as in heat treating metal parts.
Another problem with membrane separators is that the concentration of oxygen in the nitrogen product stream is not constant from moment to the next moment, that is, if the setpoint or target oxygen concentration in the nitrogen stream is 2 volume percent, the concentration of oxygen may actually vary over time from 1.8 volume percent or lower and up to 2.2 volume percent or higher.
The residual oxygen in a nitrogen gas stream outlet from a membrane generator may be removed by combustion with methane or some other hydrocarbon within a chemical reactor containing a hot active catalyst which is capable of facilitating the reaction of oxygen with the hydrocarbon, thus xe2x80x9cburning outxe2x80x9d or combusting the residual oxygen. However, even when using the best operating parameters, burning the residual oxygen with a hydrocarbon may produce other impurities such as carbon dioxide, carbon monoxide, and water vapor. This process is well known and is described in U.S. Pat. No. 5,242,509 to Rancon et al.
The carbon dioxide and water vapor impurities are relatively inert and, thus, are not objectionable in many subsequent uses of this purified nitrogen gas mixture. The carbon monoxide impurity, however, is a reducing agent and, potentially, a poisonous contaminant. This problem may be a severe impediment to the use of purified nitrogen produced by burning residual oxygen impurities out of a contaminated source of nitrogen. Moreover, if the feed to the chemical reactor is the nitrogen product stream from a membrane purifier, as stated previously the oxygen content of the nitrogen stream varies from moment to moment, and thus the carbon monoxide impurity will vary from zero to some positive value.
Consequently, several processes have been developed to remove carbon monoxide from a gas such as air, argon, or nitrogen. For example, U.S. Pat. No. 3,758,666 to Frevel et al. discloses a process for removing carbon monoxide from air by initial adsorption and subsequent oxidation to carbon dioxide on the surface of a catalyst. The catalyst comprises metallic palladium on an alumina support.
Another example of such a process is described in U.S. Pat. No. 4,808,394 to Kolts et al. This patent discloses the use of a catalyst which facilitates the oxidation of carbon monoxide with free oxygen to carbon dioxide. The disclosed catalyst is reduced platinum and/or palladium on an alumina support.
Other patents describe a process of reacting carbon monoxide with oxygen gas to produce carbon dioxide using different types of catalysts. For example, U.S. Pat. No. 4,991,181 to Upchurch et al. discloses a catalyst containing a platinum group metal (in a reduced condition) and a reducible metal oxide. The metallic element in these reducible metal oxides may be tin, titanium, manganese, copper, or cesium.
In all of the above processes, the noble metal catalytic component was in a reduced or metallic state. Therefore, it was necessary to add oxygen gas to the feed gas stream in order to promote the conversion of carbon monoxide to carbon dioxide. The oxygen source was either in the starting gas stream itself or, more typically, from an outside gas stream. In addition to oxygen gas, these processes require hydrogen to regenerate (or reduce) the platinum or palladium component of the catalyst. Obviously, the addition of oxygen and hydrogen gases increases the cost of such processes.
In view of the above prior art processes, a need exists in the art for an efficient and economical membrane purification process to produce a high purity inert gas stream which is free of undesirable impurities such as oxygen and carbon monoxide. More particularly, a need exists in the art for a process that effectively removes carbon monoxide from an inert gas stream without the need for using hydrogen or an added oxygen gas, and without regard to the flow rate of the inert gas.
In accordance with the present invention, apparatus and process are provided for purifying a stream of nitrogen or another inert gas contaminated with carbon monoxide so as to minimize the reducing effect of carbon monoxide and its potential threat of toxicity.
In one aspect of the present invention, a process is presented for removing substantially all carbon monoxide from a gas stream comprising a major amount of an inert gas (preferably nitrogen) and a minor amount of carbon monoxide, which amount of carbon monoxide may be varying from substantially zero to a positive amount. In one embodiment, the process comprises contacting the gas stream with a metal oxide in a reaction zone at conditions effective to convert substantially all of the carbon monoxide to carbon dioxide and thus produce a purified gas stream substantially free of carbon monoxide. Preferably, the metal oxide is selected from the group consisting of palladium oxides, platinum oxides, and nickel oxides.
A second aspect of the present invention is an apparatus for removing carbon monoxide from a gas stream comprising a major amount of an inert gas and a minor amount of carbon monoxide. The apparatus comprises a reactor vessel, a reactor vessel gas inlet and a reactor vessel gas outlet, the reactor vessel having an internal reaction zone space at least partially filled with a metal oxide, the metal oxide present in sufficient volume for contacting the gas stream with the metal oxide at conditions effective to convert substantially all of the carbon monoxide to carbon dioxide and thus produce a purified gas stream substantially free of carbon monoxide.
Preferably the reactor vessel includes an oxidizing gas inlet and an oxidizing gas outlet, allowing the residual metal which was formerly oxidized to be regenerated and form anew the metal oxide. Particularly preferred are methods and apparatus of the invention wherein two or more reactor vessels are arranged in parallel flow relationship with respect to the gas stream from which carbon monoxide is to be removed. In the case where there are two reactor vessels in parallel, a first reactor vessel can be functioning in the carbon monoxide removal mode, while a second reactor vessel is being regenerated, as is further described herein.
The apparatus and process of the present invention provide a more efficient and economical means for removing carbon monoxide from nitrogen or another inert gas by using one or more metal oxide reactors. One advantage of the inventive apparatus and process is that means are provided for removing carbon monoxide from a feed stream of nitrogen or another inert gas without the addition of hydrogen or oxygen gas to the feed gas stream, and without regard to the flow rate of the feed gas stream. Also, it does not matter if the feed gas stream to the reactor vessel of the inventive apparatus varies in carbon monoxide concentration, thus solving a major problem with previous methods.
The product gas stream from the process and apparatus of the present invention has a variety of uses, including heat treatment atmospheres, electronics packaging atmospheres for processes such as wave soldering and reflow soldering, as well as others which may be envisioned by those skilled in the art.
These and other objects of the present invention will become apparent after reviewing the following description of preferred embodiments and the appended drawing.